Energy-efficient, laser-based method and system for processing target material

ABSTRACT

An energy-efficient method and system for processing target material such as microstructures in a microscopic region without causing undesirable changes in electrical and/or physical characteristics of material surrounding the target material is provided. The system includes a controller for generating a processing control signal and a signal generator for generating a modulated drive waveform based on the processing control signal. The waveform has a sub-nanosecond rise time. The system also includes a gain-switched, pulsed semiconductor seed laser for generating a laser pulse train at a repetition rate. The drive waveform pumps the laser so that each pulse of the pulse train has a predetermined shape. Further, the system includes a laser amplifier for optically amplifying the pulse train to obtain an amplified pulse train without significantly changing the predetermined shape of the pulses. The amplified pulses have little distortion and have substantially the same relative temporal power distribution as the original pulse train from the laser. Each of the amplified pulses has a substantially square temporal power density distribution, a sharp rise time, a pulse duration and a fall time. The system further includes a beam delivery and focusing subsystem for delivering and focusing at least a portion of the amplified pulse train onto the target material. The rise time (less than about 1 ns) is fast enough to efficiently couple laser energy to the target material, the pulse duration (typically 2-10 ns) is sufficient to process the target material, and the fall time (a few ns) is rapid enough to prevent the undesirable changes to the material surrounding the target material.

CROSS REFERENCE TO RELATED PATENT AND APPLICATION

This application is a divisional of U.S. Ser. No. 11/305,129 (i.e., the'129 application) filed Dec. 19, 2007, which is a continuation of U.S.Ser. No. 10/818,920 (i.e., the '920 application), filed Apr. 6, 2004,which is a continuation of Ser. No. 09/941,389 (i.e., the '389application), filed Aug. 28, 2001, which is a continuation of U.S. Ser.No. 09/473,926 (i.e., the '926 application) filed Dec. 28, 1999, nowU.S. Pat. No. 6,281,471. The entire contents of the '920, '389, and '926applications are incorporated herein by reference. This application isrelated to U.S. Ser. No. 09/156,895, filed Sep. 18, 1998, now U.S. Pat.No. 6,144,118, entitled “High Speed Precision Positioning Apparatus”.This application is also related to U.S. Pat. No. 5,998,759 (i.e, the'759 patent) entitled “Laser Processing”, having the same assignee asthe present invention. The entire disclosure of the '759 patent isincorporated herein by reference. This application is also related toU.S. Ser. No. 11/305,129 filed Dec. 19, 2005.

TECHNICAL FIELD

This invention relates to energy-efficient, laser-based methods andsystems for processing target material. In particular, this inventionrelates to the use of a pulsed laser beam to ablate or otherwise alter aportion of a circuit element on a semiconductor substrate, and isparticularly applicable to vaporizing metal, polysilicide andpolysilicon links for memory repair. Further application can be found inlaser-based micromachining and other repair operations, particularlywhen it is desired to ablate or modify a microscopic structure withoutdamaging surrounding areas and structures, which often havenon-homogeneous optical and thermal properties. Similarly, the materialprocessing operations can be applied to other microscopic semiconductordevices, for instance microelectromechanical machines. Medicalapplications may also exist, such as microscopic tissue or cell ablationwith miniature fiber optic probes.

BACKGROUND OF THE INVENTION

Semiconductor devices such as memories typically have conductive linksadhered to a transparent insulator layer such as silicon oxide, which issupported by the main silicon substrate. During laser processing of suchsemiconductor devices, while the beam is incident on the link or circuitelement, some of the energy also reaches the substrate and otherstructures. Depending upon the power of the beam, length of time ofapplication of the beam, and other operating parameters, the siliconsubstrate and/or adjacent can be overheated and damaged.

Several prior art references teach the importance of wavelengthselection as a critical parameter for substrate damage control. U.S.Pat. Nos. 4,399,345, 5,265,114, 5,473,624, 5,569,398 disclose thebenefits of wavelength selection in the range beyond 1.2 um to avoiddamaging silicon substrates.

The disclosure of the above-noted '759 patent further elaborates on thewavelength characteristics of silicon. The absorption in silicon rapidlydrops off after about one micron with an absorption edge of about 1.12microns at room temperature. At wavelengths greater than 1.12 microns,the silicon starts to transmit more and more easily and, thus, it ispossible to obtain better part yields upon removing material from thesilicon. In the range around 1 micron the absorption coefficientdecrease by a factor of four orders of magnitude going from 0.9 micronsto 1.2 microns. In going from the standard laser wavelength of 1.047microns to 1.2 microns the curve shows a drop of two orders ofmagnitude. This shows a drastic change in absorption for a very slightchange in wavelength. Thus, operating the laser at a wavelength beyondthe absorption edge of the substrate circumvents damage to thesubstrate, which is especially important if there is a slightmisalignment of the laser beam with respect to the link or where thefocused spot extends beyond the link structure. Furthermore, if thesubstrate temperature rises during processing the absorption curveshifts will shift further into the infrared which can lead to thermalrunaway conditions and catastrophic damage.

The problem of liquid crystal repair is similar to the problem of metallink ablation. The wavelength selection principle for maximizingabsorption contrast was advantageously applied in the green wavelengthregion in a manner analogous to the above disclosures for the samepurpose—namely removal of metal without substrate damage. The systemmanufactured by Florod is described in the publication “Xenon LaserRepairs Liquid Crystal Displays”, LASERS AND OPTRONICS, pages 39-41,April 1988.

Just as wavelength selection has proven to be advantageous, it has beenrecognized that other parameters can be adjusted to improve the laserprocessing window. For example, it was noted in “Computer Simulation ofTarget Link Explosion in Laser Programmable Redundancy for SiliconMemory” by L. M. Scarfone and J. D. Chlipala, 1986, p. 371, “It isdesirable that laser wavelengths and various material thicknesses beselected to enhance the absorption for the link removal process andreduce it elsewhere to prevent damage to the remainder of thestructure.” The usefulness, in general, of thicker insulative layersunderneath links or circuit elements and the usefulness of limiting theduration of heating pulses has also been recognized, as in the paperco-authored by the applicant, “Laser Adjustment of Linear MonolithicCircuits”, Litwin and Smart, 100/L.I.A., Vol. 38, ICAELO (1983).

The '759 patent teaches the tradeoffs that exist with selection of thelonger wavelengths—specifically compromises with respect to spot size,depth of focus, and pulse width, available from Nd:YAG lasers. Theseparameters are of critical importance for laser processing atincreasingly fine dimensions, and where the chances of collateral damageto surrounding structures exist.

In fact, any improvement which widens the processing window isadvantageous as the industry continues to push toward higher densitymicrostructures and the associated geometries which are a fraction ofone micron in depth or lateral dimension. The tolerances of energycontrol and target absorption become large compared to the energyrequired to process the microstructure at this scale. It should be notedfrom the above discussion that laser processing parameters are notnecessarily independent in micromachining applications where a smalllaser spot, about 1 μm, is required. For instance, the spot size andpulse width are generally minimized with short wavelengths, say lessthan 1.2 μm, but the absorption contrast is not maximized. Makers ofsemiconductor devices typically continue production of earlier developedproducts while developing and entering production of more advancedversions that typically employ different structures and processes. Manycurrent memory products employ polysilicide or polysilicon links whilesmaller link structures of metal are used for more advanced productssuch as the 256-megabit memories. Links of 1 micron width, and ⅓ microndepth, lying upon a thin silicon oxide layer of 0.3 to 0.5 microns arebeing used in such large memories. Production facilities traditionallyhave utilized Q-switched diode pumped YAG lasers at and relatedequipment capable of operating at the conventional wavelengths of 1.047μm-1.32 μm and related equipment capable of operating in the wavelengthregion recognized for its lower absorption by silicon. However, theseusers also recognize the benefits of equipment improvements whichresults in clean severing of link structures without the risk of laterchip failures due to conductive residue or contamination near theablation site.

Other degrees of freedom include laser pulse energy density (deliveredto the target) and pulse duration. It has been taught in the prior artthat pulse width should be limited to avoid damage in micromachiningapplications. For example, in U.S. Pat. No. 5,059,764 a laser processingworkstation is disclosed wherein a q-switched laser system is utilizedto produce, among other things, relatively short pulses on the order of10-50 ns. It was disclosed that for material processing applications(like semiconductor memory repair via link blowing and precisionengraving), the output pulse width should be relatively short—and that apulse width less than 50 ns is required in many applications, forexample 30 ns. The proper choice of pulse width allows for ablation(evaporation without melting).

High speed pulsed laser designs may utilize Q-switched, gain switched,or mode-locked operation. The pulse duration and shape of standardQ-switched and other pulsed lasers can be approximated at a fundamentallevel by integrating the coupled rate equations describing thepopulation inversion and the photon number density relative to thelasing threshold at the start of the pulse. For the Q-switched case, ona normalized scale, a higher number of atoms in the inverted populationrelative to the threshold the faster the pulse rise time, the narrowerthe width, and the higher the peak energy. As the ratio decreases thepulse shape becomes broader with lower energy concentration.

Often Q-switched laser pulses resemble a Gaussian temporal distribution,or a mixture of a Gaussian with an exponential decaying tail. Asdisclosed in the '759 patent, the shorter wavelength diode pumpedsystems are capable of producing relatively short pulses, about 10 ns,when measured at the half power points (i.e., standard definition ofpulse duration) and are operated in a favorable wavelength region.Despite successful operation, applicant has found several limitationsassociated with the temporal pulse shape characteristic of standarddiode pumped Q-switch laser systems, including the practical rise timelimitations, the power distribution between the half maximum points, andthe pulse decay characteristic which, when improved using the method andsystem of the present invention, provided noticeably better results in ametal link blowing application.

Throughout the remainder of this specification, “pulse shaping” refersto the generation of a laser pulse which is to be detected with adetector of electromagnetic radiation where “shape” refers to the poweron the detector as a function of time. Furthermore, “pulse width” or“pulse duration” refers to the full width at half maximum (FWHM) unlessotherwise stated. Also, Q-switched pulses collectively refers totemporal distribution of pulses obtained, for example, in standardQ-switched systems which may resemble a mixture of a substantiallyGaussian central lobe with a relatively slow decaying exponential tail.These wave shapes are formally referred to as a “Q-switched pulseenvelope” in laser literature. FIG. 1 c shows such pulses.

In U.S. Pat. No. 5,208,437 (i.e., the '437 patent), a pulse widthspecification of less than 1 ns was specified for a memory repairapplication. Earlier work by the co-inventors of the '437 patentdisclosed in “Laser Cutting of Aluminum Thin Film With No Damage toUnder Layers”, ANNALS OF THE CIRP, Vol 28/1, 1979, included experimentalresults with relatively short laser pulses having a “Gaussian” shape asdefined above. The results indicated a “desired portion of theinterconnection pattern,” which is made of aluminum or the like, “can becut without the layer disposed below the interconnection pattern beingdamaged”. Specifications for the pulse width of substantially 1 ns orless with energy density of substantially 10⁶ W/cm² were disclosed forthe apparatus. However, there was no disclosure regarding a method oftemporal pulse shaping, although spatially the beam was shaped tocorrespond to the interconnection pattern. Furthermore, applicant'sanalysis on high density memory devices having multiple layers withspecified pulsewidths in the ultrafast range, which is approached withthe 100-300 ps used in the '437 patent, have not been satisfactory.Overcoming this limitation would presently require the ultrafast lasersystem to produce multiple pulses for processing each target site whichwould slow the laser processing rate to an unacceptable level.

Continuing to the ultrafast scale, experimental results have beendisclosed for micromachining operations. The ultrafast pulses havedurations on the order of fs (10-15 sec) to ps (10-12) and, at thedecreased scale, exploit material properties at the atomic and molecularwhich are fundamentally different than found in the range of severalhundred ps to ns.

In U.S. Pat. No. 5,656,186 and the publication “Ultrashort Laser Pulsestackle precision Machining”, LASER FOCUS WORLD, August 1997, pages101-118, machining operations at several wavelengths were analyzed, andmachined feature sizes significantly smaller than the diffractionlimited spot size of the focused beam were demonstrated.

Laser systems for ultrafast pulse generation vary in complexity and areexemplary embodiments are described in U.S. Pat. Nos. 5,920,668 and5,400,350, and in Ultrafast Lasers Escape The Lab”, PHOTONICS SPECTRA,July 1998, pp. 157-161. The embodiments generally include methods forpulse stretching of mode locked ultrafast pulses prior to amplificationto avoid amplifier saturation followed by compression to extremelynarrow widths. This technology holds promise for certain class ofmicromachining and possibly finer scale “nanomachining” operations, thelatter benefit afforded by machining below diffraction limit. However,Applicant has discovered practical limitations at the present time withthe available power in each pulse for applications like metal linkblowing and similar micromachining applications leading to theunacceptable requirement for multiple pulses.

Applicant wishes to elaborate on the rationale for the use of a shortpulse, fast rise time pulse is indicated in the following paragraphs asthe reasons are manifold and a number of theoretical and empiricalpapers and books have been written on the subject. Ablation of metallinks is taken as an example, although the principles extend to manylaser processing applications where a target material is surrounded bymaterial having substantially different optical and thermal properties.The following references 1-3 are examples:

-   1. John F. Ready, Effects of High Power Laser Radiation, ACADEMIC    PRESS, New York 1971, pages 115-116.-   2. Sidney S. Charschan, Guide for Material Processing By Lasers,    Laser Institute of America, The Paul M. Harrod Company, Baltimore    Md., 1977, pages 5-13.-   3. Joseph Bernstein, J. H. Lee, Gang Yang, Tariq A. Dahrnas,    Analysis of Laser Metal-Cut Energy Process Window (to be published).    Metal Reflectivity

Metal reflectivity decreases with increased power density of a laserpulse (ref. 1). The reflectivity of a metal is directly proportional tothe free electron conductivity in a material. At high electric fielddensities as delivered by a high intensity laser, the collision timebetween electrons and the lattice is reduced. This shortening of thecollision time reduces the conductivity and hence the reflectivity. Forexample, the reflectivity of aluminum decreases from 92% to less than25% as the laser power densities increases to the range of 10⁹watts/cm². Hence, to circumvent the loss of laser energy to reflectionit is advantageous to achieve high power density at the work piece in asshort a time as possible.

Thermal Diffusivity

The distance D that heat travels during a laser pulse is proportional tothe laser pulse width as follows:D=√{square root over (kt)}where:K is the thermal diffusivity of the material; andt is the length of the laser pulse.

Hence, it can be seen that a short laser pulse prevents heat dissipatingto the substrate below the melting link and also heat conductinglaterally to the material contiguous to the link. However the pulse mustbe long enough to heat the link material all the way through.

Thermal Stress and Link Removal

Through the absorption of the laser energy the target metal link heatsup and tries to expand. However, the oxide surrounding the link containsthe expanding material. Hence, stress is built up within the oxide. Atsome point the pressure of the expanding metal exceeds the yield pointof the oxide and the oxide cracks and the metal link explodes into afine particle vapor. The principal crack points of metal link occurs atthe maximum stress points, which are at the edges of the link both topand bottom as shown in FIG. 1 b.

If the oxide over the link is somewhat thin then the cracking of theoxide will occur at the top of the link only and the oxide and link willbe removed cleanly as shown in FIG. 1 a. However, if the oxide issomewhat thick, cracking can occur at the bottom of the link as well asthe top and the crack will propagate down to the substrate as shown inFIG. 1 b. This is a highly undesirable circumstance.

Q-switched laser systems can be modified to provide short pulses ofvarious shapes. Typical prior art lasers that produce high peak power,short pulse lasers are standard Q-switched lasers. These lasers producea temporal pulse having a moderate pulse rise time. It is possible tochange this temporal shape by using a Pockels Cell pulse slicer thatswitch out sections of the laser beam. In U.S. Pat. No. 4,483,005 (i.e.,the '005 patent), invented by the Applicant of the present invention andhaving the same assignee, various methods for affecting (i.e., reducing)laser beam pulse width are disclosed. As taught in the '005 patent,which is hereby incorporated by reference, the laser pulse can be shapedsomewhat to produce a “non-Gaussian” shaped beam by truncating energyoutside the central lobe. It should be noted that if a relatively broadQ-switched waveform is to be transformed to a narrow, uniform shape,only a small fraction of the pulse energy will be used. For example,truncation of a Gaussian pulse to provide a sharp rise time and a narrowpulse with flatness to within 10% reduces the pulse energy by about 65%.

Similarly, in U.S. Pat. No. 4,114,018 (the '018 patent), temporal pulseshaping to produce square pulses is disclosed. FIG. 7 shows the timeinterval for relatively flat laser power output. In the '018 patentedmethod, it is necessary to remove a temporal segment of the beamintensity in order to generate the desired pulses.

A desirable improvement over the prior art would provide an efficientmethod for generating short pulses with high energy enclosure within thepulse duration with rapidly decaying tails. In order to accomplish this,laser technology which produces pulse shapes different than those of theQ-switched pulse envelope is preferred. Such pulses have fast rise time,uniform energy in the central lobe, and fast decay.

The fast rise-time, high power density pulse as produced by a laserother than a standard Q-switched Nd:YAG will best accomplish this task.

These benefits are implemented in a preferred manner in a system whichuses laser technology departing significantly from the traditionalQ-switched, solid state diode or lamp pumped, YAG technology.

Improvements over the prior art are desired with a method and system forgenerating pulses having a shape which is different than standardQ-switched pulses—pulses having faster rise time, relatively uniform andhigher energy concentration in the central lobe, and fast fall time.

SUMMARY OF THE INVENTION

Applicant has determined that improved results can be obtained inapplications of metal link blowing. For instance, a non-Gaussian,substantially rectangular pulse shape is particularly advantageous formetal link processing where an overlying insulator exists. Applicantsresults show that the fast rise time on the order of 1 ns, andpreferably about 0.5 ns, provides a thermal shock to the overlying layerof oxide which facilitates the link blowing process. In addition, at thehigher power density the reflectivity is reduced with the fast risingshort pulse. A pulse duration of about 5 ns with a substantially uniformpulse shape allows more energy to be coupled to the link leading to areduced energy requirement for link removal. Rapid fall time of about 2ns is important to eliminate the possibility of substrate damage.Furthermore, an advantage of a nearly square power density pulse in timeis that the power density is the highest when it is needed and the pulseis off when it is not.

A short fast rising pulse will allow the top of the link to melt andexpand first before the heat can diffuse down to the lower portion ofthe link. Hence, stress is built up in the top of the link and promotescracking of the top layer without generating a crack down to thesubstrate.

It is an object of this invention to provide a compact, gain switchedlaser system which has the capability for generating sub-nanosecond risetime pulses having short duration of a few nanoseconds and rapid falltime. State of the art fast pulse systems incorporate gain switchedtechnology, in which a low power semiconductor seed laser is rapidly anddirectly modulated to produce a controlled pulse shape which issubsequently amplified with a laser amplifier, such as a cladding pumpedfiber optic system with a high power laser diode or diode array used asthe pump laser. Such laser systems are described in U.S. Pat. No.5,694,408 and PCT Application No. PCT/US98/42050, and are “buildingblocks” of certain ultrafast chirped pulse amplifier systems, forinstance the system described in U.S. Pat. No. 5,400,350.

It is a general object of the invention to improve upon prior art laserprocessing methods and systems, particularly those where the opticaland/or thermal properties of a region near the target material differsubstantially.

It is a general object of the invention to provide laser pulse shapingcapability for micromachining and laser material processingapplications, for instance laser ablation of links or otherinterconnects on semiconductor memories, trimming, drilling, marking,and micromachining. A predetermined waveform shape is generated from again-switched laser which is different than that of the standardQ-switched systems.

It is an object of the invention to provide improvements and margin forsemiconductor processing, for example, 16-256 megabit semiconductorrepair, which results in clean processing of microstructures without therisk of later device failure due to conductive residue or contaminationnear the ablation site.

It is an object of the invention to provide a pulse waveform rise timein as short as a few hundred picoseconds, the pulse duration typicallyless than about 10 nanoseconds with rapid pulse decay, thereby providinglaser processing of a target structure at high power density, wherebydamage arising from thermal shock and diffusion in the surroundingregions is minimized.

It is an object of the invention to prevent damage to the structuressurrounding and beneath the target material in semiconductor laserprocessing applications by achieving high power density at the workpiecein a very short time with a high power, fast rise time pulse at anywavelength suitable for the laser ablation process thereby improving theprocess window in a semiconductor material processing application.

It is an object of the invention to process a target site with a singlelaser processing pulse with rise time fast enough and with sufficientpower density so as to provide a reduction in the reflectivity of ametal target structure, such a single metal link on a semiconductormemory, and hence provide more efficient coupling of the laser energy.The fast rising laser pulse is of sufficient pulse duration toefficiently heat and vaporize the material of each metallic targetstructure with relatively uniform power density during the ablationperiod, yet a rapid pulse fall time after the target material isvaporized avoids damage to surrounding and underlying structures.

It is an object of the invention to provide superior performance insemiconductor metal link blowing applications when compared to systemsutilizing standard Q-switched lasers, such lasers having typical pulserise times of several nanoseconds and represented by a Q-switched pulseenvelope. A laser pulse is generated to provide a substantially squarepulse shape with pulse duration in the range of about 2-10 nanosecondsand a rise time of about 1 ns and preferably about 0.4 ns. Additionally,the pulse decay is to be rapid when switched off thereby allowing only avery small fraction of pulse energy to remain after the predeterminedpulse duration, the pulse “tails” rapidly decaying to a sufficiently lowlevel so as to avoid the possibility of damaging the underlyingsubstrate or other non-target materials. A comparison of these pulses isillustrated in FIG. 2.

It is an object of the invention to expand the processing window of asemiconductor laser ablation process to provide rapid and efficientablation of microscopic structures surrounded by materials havingdifferent optical and thermal properties. Such structures are typicallyarranged in a manner where the width and spacing between the structuresis about 1 micron or smaller and stacked in depth. The application of ashort laser pulse cleanly ablates the target material, yet damage tosurrounding materials caused by heat dissipation in either the lateraldirection or damage to the underlying substrate below the targetmaterial is prevented.

It is an object the invention to controllably machine a material havingsubstantially homogeneous optical and thermal properties with theapplication of a short pulse having high energy density, the pulseduration being a few nanoseconds in the material processing range wherea fluence threshold is approximately proportional to the square root oflaser pulse width.

In carrying out the above objects and other objects of the presentinvention, an energy-efficient, laser-based method for processing targetmaterial having a specified dimension in a microscopic region withoutcausing undesirable changes in electrical or physical characteristics ofmaterial surrounding the target material is provided. The methodincludes generating a laser pulse train utilizing a laser having awavelength at a repetition rate wherein each of the pulses of the pulsetrain has a predetermined shape. The method then includes opticallyamplifying the pulse train without significantly changing thepredetermined shape of the pulses to obtain an amplified pulse train.Each of the amplified pulses has a substantially square temporal powerdensity distribution, a sharp rise time, a pulse duration and a falltime. The method also includes delivering and focusing at least aportion of the amplified pulse train into a spot on the target materialwherein the rise time is fast enough to efficiently couple laser energyto the target material, the pulse duration is sufficient to process thetarget material and the fall time is rapid enough to prevent theundesirable changes to the material surrounding the target material.

The target material may include microstructures such as conductive linesor links, the latter being common circuit elements of redundantsemiconductor memories. The conductive lines may be metal lines andwherein the pulse duration is sufficient to effectively heat andvaporize the metal lines, or a specified portion thereof.

The target material may be a part of a semiconductor device such as asemiconductor memory having 16-256 megabits.

At least a portion of the material surrounding the target material maybe a substrate such as a semiconductor substrate.

The target material may be part of a microelectronic device.

The substantially square temporal power density distribution issufficient to substantially completely ablate the target material.

Preferably, the rise time is less than 1 nanosecond and, even morepreferably, is less than 0.5 nanoseconds.

Preferably, the pulse duration is less than 10 nanoseconds and, evenmore preferably, is less than 5 nanoseconds.

Also, preferably, the fall time is less than 2 nanoseconds.

A single amplified pulse is typically sufficient to process the targetmaterial.

The target material may have a reflectivity to the amplified pulses andwherein the power density of the amplified pulses is sufficiently highto reduce the reflectivity of the target material to the amplifiedpulses and to provide efficient coupling of the laser energy to thetarget material.

Preferably, each amplified pulse has a relatively uniform power densitydistribution throughout the pulse duration.

Preferably, each pulse has a temporal power density distribution uniformto within ten percent during the pulse duration.

The material surrounding the target material may have opticalproperties, including absorption and polarization sensitivity, andthermal diffusivity properties different from the correspondingproperties of the target material.

Preferably, the repetition rate is at least 1000 pulses/second and eachof the amplified pulses has at least 0.1 and up to 3 microjoules ofenergy.

Preferably, the step of optically amplifying provides a gain of at least20 DB.

Also, preferably, both the rise time and the fall time are less thanone-half of the pulse duration and wherein peak power of each amplifiedpulse is substantially constant between the rise and fall times.

Preferably, each of the amplified pulses has a tail and the method alsoincludes attenuating laser energy in the tails of the amplified pulsesto reduce fall time of the amplified pulses while substantiallymaintaining the amount of power of the pulses.

Still further in carrying out the above objects and other objects of thepresent invention, an energy-efficient system for processing targetmaterial having a specified dimension in a microscopic region withoutcausing undesirable changes in electrical or physical characteristics ofmaterial surrounding the target material is provided. The systemincludes a controller for generating a processing control signal and asignal generator for generating a modulated drive waveform based on theprocessing control signal. The waveform has a sub-nanosecond rise time.The system also includes a gain-switched, pulsed seed laser having awavelength for generating a laser pulse train at a repetition rate. Thedrive waveform pumps the laser so that each pulse of the pulse train hasa predetermined shape. Further, the system includes a laser amplifierfor optically amplifying the pulse train to obtain an amplified pulsetrain without significantly changing the predetermined shape of thepulses. Each of the amplified pulses has a substantially square temporalpower density distribution, a sharp rise time, a pulse duration and afall time. The system further includes a beam delivery and focusingsubsystem for delivering and focusing at least a portion of theamplified pulse train onto the target material. The rise time is fastenough to efficiently couple laser energy to the target material, thepulse duration is sufficient to process the target material, and thefall time is rapid enough to prevent the undesirable changes to thematerial surrounding the target material.

The laser amplifier preferably includes an optical fiber and a pump suchas a laser diode to pump the optical fiber wherein the pump is distinctfrom the seed laser.

The laser diode pump source may also be gain switched (pulsed anddirectly modulated) to increase diode lifetime by switching to the “off”state during extended periods where laser processing is not occurring.

Preferably, the seed laser includes a laser diode.

The system may include an attenuator for attenuating laser energy in thetails of the amplified pulses to reduce fall time of the amplifiedpulses while substantially maintaining the amount of energy of thepulses.

The pulse duration may be chosen as a function of a specified targetmaterial dimension. The specified material dimension may be less thanthe laser wavelength. The preferred laser is a high speed, semiconductorlaser having a wavelength less than about 2 μm. Future material advancesin semiconductor laser diode technology and fiber materials may providefor operation in the visible region as well as at longer infraredwavelengths.

The seed laser diode may be a multimode diode laser or a singlefrequency (single mode) laser utilizing a distributed Bragg reflector(DBR), distributed feedback (DFB), or an external cavity design.

The spot size typically has a dimension in the range of about 1 μm-4 μm.

The density of the memory may be at least 16-256 megabits.

The semiconductor device may be a microelectromechanical device.

Preferably, the attenuated laser energy in the pulse tail is attenuatedby at least 10 dB within 1.5 times the pulse duration.

Yet still further in carrying out the above objects and other objects ofthe present invention, an energy-efficient, laser-based method forablating a metal link having a specified dimension embedded in at leastone passivation layer without causing undesirable changes in electricalor physical characteristics of the at least one passivation layersurrounding the metal link is provided. The method includes generating alaser pulse train utilizing a laser having a wavelength at a repetitionrate. Each of the pulses of the pulse train has a predetermined shape.The method also includes optically amplifying the pulse train withoutsignificantly changing the predetermined shape of the pulses. to obtainan amplified pulse train. Each of the amplified pulses has asubstantially square temporal power density distribution, a sharp risetime, a pulse duration and a fall time. The method further includesdelivering and focusing at least a portion of the amplified pulse traininto a spot on the metal link. The rise time is fast enough toefficiently couple laser energy to the metal link. The pulse duration issufficient to ablate the metal link and the fall time is rapid enough toprevent the undesirable changes to the at least one passivation layersurrounding the metal link.

Still further in carrying out the above objects and other objects of thepresent invention, an energy-efficient system for ablating a metal linkhaving a specified dimension embedded in at least one passivation layerwithout causing undesirable changes in electrical or physicalcharacteristics of the at least one passivation layer surrounding themetal link is provided. The system includes a controller for generatinga processing control signal and a signal generator for generating amodulated drive waveform based on the processing control signal. Thewaveform has a sub-nanosecond rise time. The system also includes again-switched, pulsed seed laser having a wavelength for generating alaser pulse train at a repetition rate. The drive waveform pumps thelaser so that each pulse of the pulse train has a predetermined shape.Further, the system includes a laser amplifier for optically amplifyingthe pulse train without significantly changing the predetermined shapeof the pulses to obtain an amplified pulse train. Each of the amplifiedpulses has a substantially square temporal power density distribution, asharp rise time, a pulse duration and a fall time. The system furtherincludes a beam delivery and focusing subsystem for delivering andfocusing at least a portion of the amplified pulse train into a spot onthe metal link. The rise time is fast enough to efficiently couple laserenergy to the metal link. The pulse duration is sufficient to ablate themetal link, and the fall time is rapid enough to prevent the undesirablechanges to the at least one passivation layer surrounding the metallink.

The metal link may be embedded in a top passivation layer thereover anda bottom passivation layer thereunder. The pulse duration is sufficientto crack the top passivation layer but not the bottom passivation layer.

In carrying out the above objects and other objects of the invention amethod is provided to ablate target material using a laser having awavelength suitable for laser material processing while avoiding damageto surrounding materials. The method includes the steps of modulating alaser beam to produce a predetermined gain-switched pulse and focusingthe laser beam onto the target region. The predetermined gain-switchedpulse shape includes a rise time of the laser pulse fast enough toefficiently couple laser energy to a target structure, with a pulseduration of sufficient length to efficiently heat and vaporize thetarget material, and a pulse decay time which is rapid enough to avoiddamage of structures surrounding the target material.

Further in carrying out the above objects and other objects of theinvention a system for is provided to ablate material using a laserhaving a wavelength suitable for laser processing while avoiding damageto surrounding materials. The system includes a laser source, componentsto modulate the laser source to generate a laser pulse having apredetermined gain-switched pulse shape, and optical components forfocusing the laser beam onto the target region. The predetermined pulseshape includes an optical rise time of the laser pulse fast enough toefficiently couple laser energy to a target structure, with a pulseduration of sufficient length to efficiently heat and vaporize thetarget material, and a pulse decay time which is rapid enough to avoiddamage of structures surrounding the target material.

In a preferred construction of the invention, the gain-switched pulseshape includes a fast rise time pulse, substantially flat at the top,with a fast pulse fall time. A “seed” laser diode is directly modulatedto generate a predetermined pulse shape. The optical power is increasedthrough amplification with a fiber laser amplifier to output powerlevels sufficient for laser processing. The resulting gain-switchedpulse at the fiber laser amplifier output is focused onto the targetregion.

In a construction of the invention, it can be advantageous to directlymodulate the “seed” diode to produce a predetermined gain-switchedsquare pulse and provide low distortion amplification using a fiberlaser amplifier to provide output pulse levels sufficient for materialprocessing.

In an alternative construction, the pulse temporal power distribution ofthe directly modulated seed diode is modified to compensate fordistortion or non-uniformity of the fiber amplifier or other components,for instance the “smooth” rise of an output modulator. The resultinglaser processing pulse which is focused into the target region will havea desired shape: fast rise time, relatively flat during the pulseduration, with rapid decay.

In a construction of the invention it can be advantageous to enhance theperformance of the laser processing system by providing a “pulseslicing” module which is used to attenuate laser energy remaining at theoutput of the laser processing system when the “seed” laser pulse isterminated, thereby preventing heating of sensitive structures notdesignated as target material after processing is complete. The “pulseslicing” technique is useful to attenuate the tail of either a modifiedpulse or a standard Q-switched pulse. This is illustrated in FIGS. 4 aand 4 b, wherein a log scale is provided in the vertical axis of FIG. 4b.

It is preferred to perform laser processing operations, particularlymetal link blowing, at pulse rates of at least 1 KHz (1000pulses/second) with laser pulse energy of at least 0.1 microjoules in apulse, the 0.1 microjoules being emitted at the output of the fiberamplifier, where the fiber optic amplifier gain is at least 20 DB(1000:1).

In a construction of the invention, a laser pulse is shaped having arise and fall time shorter than about one-half of the pulse duration andwhere the peak power is approximately constant between the rise and falltime.

In a construction of the invention, it is possible to generate a seriesof closely-spaced, short pulses which, when combined, produce a desiredpulse shape as illustrated in FIGS. 3 a and 3 b.

In a construction of a system using the invention it can also beadvantageous to operate the laser at pulse repetition rates exceedingthe material processing rate and utilize a computer controlled opticalswitch to select processing pulses, the computer being operativelyconnected to a beam positioning system used to position a focused laserbeam for material processing.

The above objects and other objects, features, and advantages of thepresent invention are readily apparent from the following detaileddescription of the best mode for carrying out the invention when takenin connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a shows schematically stress cracks in a top surface layer onlyof a semiconductor caused by expanding vaporized metal;

FIG. 1 b shows schematically stress cracks in top and bottom layers of asemiconductor caused by expanding vaporized metal;

FIG. 1 c shows typical prior art laser pulses resembling a Gaussianshape, or a mixture of a Gaussian with an exponential tail, referred toas a “Q-switched pulse envelope”;

FIG. 2 shows the preferred pulse shape of the present invention forprocessing metal links when compared to a Q-switched of the same totalenergy;

FIGS. 3 a and 3 b show a method of combining two short pulses closelyspaced in time to create a modified pulse;

FIGS. 4 a and 4 b show the result of “pulse slicing” for improving thepulse energy enclosure of a general pulse shape;

FIG. 5 is a general block diagram of a preferred laser system for lasermaterial processing;

FIG. 6 a is a schematic diagram of one type of a MOPA laser system witha distributed Bragg laser as the semiconductor seed laser; this is asingle mode laser and a fiber optic amplifier producing the preferredpulse shape;

FIG. 6 b is a schematic diagram of a single frequency laser withexternal cavity tuning and a fiber optic amplifier;

FIG. 7 is a block diagram schematic of another laser system of thepresent invention including a preferred attenuator and an optionalshifter;

FIG. 8 is a graph of temperature at the interface between the silicondioxide layer and the silicon substrate in FIG. 9, as a function of thethickness of the silicon dioxide layer;

FIG. 9 shows a perspective diagrammatic view of a link of a memory onits substrate;

FIG. 10 is a drawing of a Gaussian laser beam focused onto a small spoton a focal plane containing a metal link emphasizing the microscopicsize of the link compared to the diffraction-limited beam waist; and

FIGS. 11 a and 11 b are graphs which show the results of a computerfinite element analysis simulation where the time history of stress andtemperature is plotted in the graphs for a Q-switched pulse and squarepulse used for metal link processing.

BEST MODE FOR CARRYING OUT THE INVENTION

Laser Processing System Architecture

Those skilled in the art can appreciate that the following embodimentcan be applied to several applications in micromachining and laserprocessing with appropriate adjustments to parameters like laser power,energy density, spot size, wavelength, pulse width, polarization andrepetition rate. The specific application to metal link blowing isdescribed for illustrative purposes.

In a preferred embodiment of FIG. 7, a seed laser 10 and a fiberamplifier are mounted on a stable platform attached to the motion system20 and the workpiece. It is very important in removing links that thebeam be positioned with accuracy of less than 3/10 of a micron. Thetiming of the laser pulse to correlate with the relative positions ofthe target and optical system is important because of the continuousmotion required in order to obtain high processing speeds.

The laser 10 is externally controlled by the computer 33 and a signalgenerator 11 and transmits its modulated beam to a focusing subsystem 12comprising high numerical aperture optics and which may further comprisea beam deflector, for instance a galvanometer mirror controlled by ascanner control via the computer 33. The system control computer 33 isalso operatively connected to a positioning mechanism or motion system20 for the system and the signal generator 11 to properly time the pulsegeneration. The laser beam must be precisely controlled so as to producea sharply focused beam, with a spot size in the range of about 1.5-4microns, at the correct location in X, Y and Z. As such, those skilledin the art of beam positioning and focusing will recognize theimportance of optics corrected to provide near diffraction and limitedperformance and precision motion control of the laser head or targetsubstrate. Depending upon the specific laser processing applicationrequirements, it may be advantageous to provide an optical system with arelatively narrow field of view to provide diffraction limited focusingand precision X,Y motion stages for beam positioning. Furthermore,various combinations of mirror motion for rapid deflection incombination with translation stages are viable.

A step and repeat table 34 can also be used to move a wafer 22 intoposition to treat each memory die 24 thereof. Those skilled in the artof beam scanning will appreciate the advantages of a mirror-based, beamdeflection system, but, as noted above, substitution of other positionmechanisms such as X, Y translation stages for movement of the substrateand/or laser head are viable alternatives for practicing the invention.For example, the substrate positioning mechanism 34 may comprise veryprecise (well below 1 micron) X, Y, Z positioning mechanisms operatingover a limited range of travel. The positioning mechanism 20 may be usedto translate the laser processing optical system components, includingthe laser, fiber amplifier, and focusing subsystem in a coarser fashion.Further details on a preferred positioning system are disclosed in theabove-noted pending U.S. patent application entitled “High SpeedPrecision Positioning Apparatus”, Ser. No. 09/156,895, filed Sep. 18,1998.

A system optical switch 13 in the form of a further acousto-opticattenuator or pockels cell is positioned beyond the laser cavity, in thelaser output beam. Under control of the computer 33, it serves both toprevent the beam from reaching the focusing system except when desired,and, when the processing beam is required, to controllably reduce thepower of the laser beam to the desired power level. During vaporizationprocedures this power level may be as little as 10 percent of the grosslaser output, depending upon operating parameters of the system andprocess. The power level may be about 0.1 percent of the gross laseroutput during alignment procedures in which the laser output beam isaligned with the target structure prior to a vaporization procedure. Theacousto-optic device is generally preferred because of the ease of use,although the delay of the pockels cell is considerably less.

In operation, the positions of the wafer 22 (or target or substrate) arecontrolled by the computer 33. Typically, the relative movement is atsubstantially constant speed over the memory device 24 on the siliconwafer 22, but step and repeat motion of the wafer is possible. The laser10 is controlled by timing signals based on the timing signals thatcontrol the motion system. The laser 10 typically operates at a constantrepetition rate and is synchronized to the positioning system by thesystem optical switch 13.

In the system block diagram of FIG. 7, the laser beam is shown focusedupon the wafer 22. In the magnified view of FIG. 9, the laser beam isseen being focused on a link element 25 of a memory circuit or device24.

For processing fine link structures, spot size requirements are becomingincreasingly demanding. The spot size requirement is typically 1.5-4microns in diameter, with peak power occurring in the center of the spotwith good conformance to a Gaussian distribution, and with lower poweroccurring at the edges. Excellent beam quality is needed, approachingdiffraction limit, with a beam quality or “m-squared factor” of about1.1 times or better typical. This “times diffraction limit” qualitystandard is well known to those skilled in the art of laser beamanalysis. Low sidelobes are also preferred to avoid optical crosstalkand the undesirable illumination of features outside the target region.

The link 25 is somewhat smaller than the spot size, thereby mandatingprecision positioning and good spot quality. A link may be, forinstance, 1 micron wide and about ⅓ micron thick. In the casedemonstrated here, the link is made of metal, and a lateral dimension(width) and thickness are smaller than the laser wavelength.

Laser System—Preferred

In a preferred embodiment a laser subsystem of FIG. 5 utilizes a masteroscillator, power amplifier (MOPA) configuration. This system produces alaser pulse that seeds an amplifier to produce a high power short risetime pulse. A seed laser is the key to producing the fast rise time,short pulse but at very low energy levels. The system requires a laseramplifier to produce enough energy to do material processing. A fiberlaser amplifier and a high-speed infrared laser diode having an outputwavelength suitable for a laser processing application is preferred.With such a system a laser can be devised that produces a laser pulse ofthe preferred shape and speed as shown in the lower part of FIG. 5. Thatis, a fast rise time pulse, square at the top and a fast fall time. Thispulse shape, in turn, provides the desired laser-material interactionresults of reduction in metal reflectivity, low diffusion of the energyinto the device and cracking of the top oxide without damage to thelower oxide.

The MOPA configuration is relatively new and pulsed versions areregarded as state of the art. The laser diode which has sub-nanosecondrise time in response to a modulating drive waveform is a starting pointin the fiber laser MOPA configuration, with the laser diode as a gainelement. The laser diode generally has multiple longitudinal modes andthe sub-system can be configured for single mode operation or otherwisetuned with bulk components at the output or, alternatively, withintegrated fiber gratings in the system.

For instance, the Littman-Metcalf grating configuration described inproduct literature by New Focus Inc., in the external cavityconfiguration, is a viable configuration. FIG. 6 b shows a schematic ofa single frequency laser with external cavity tuning and also includesan optical fiber pumped at its cladding by diode laser pump.

Other diode laser alternatives include distributed feedback lasers (DFB)and distributed Bragg lasers (DBL) which have integrated gratings andwaveguide structures, some cases with external controls allowing theuser to independently control the gain, phase, and grating filter. SeeFIG. 6 a for a DBL configuration including a coupler 50. This providesflexible mode selection and tuning capability. The laser frequencies canbe dynamically selected with a number of the configurations byadjustments of the bulk components, such as the grating and/or mirrorsof the external cavity, or, alternatively, a fixed wavelength or modechosen. The range over which the diode central wavelength can beselected is impressive overall, from less than 1 μm to about 1.3-1.5 μmor longer, the latter wavelengths corresponding to those used for fiberoptic communication.

In any case, a key element for the purpose of this invention, at a laserwavelength selected for material processing, is the rise time of the“seed” laser diode and the pulse shape. Also, a consideration for thisinvention is that the seed laser wavelength be matched to the spectralband over which the fiber optic amplifier has high gain with littlesensitivity to small wavelength changes—i.e., in the amplifier “flat”response region for maintaining excellent pulse-to-pulse power outputwith sufficient power. For Ytterbium-doped fibers, the gain is high inabout a reasonably broad wavelength band near the 1.1 μm absorption edgeof silicon. Further development in materials or integrated fibercomponents may extend the useful wavelength regions providing moreflexibility in matching the fiber emission spectrum, the seed laserwavelength and target material properties. For example, in PhotonicsSpectra, August 1997, p. 92, the results are reported for astate-of-the-art fiber laser development over a wavelength range of 1.1μm to 1.7 μm.

The operation of a Raman shifter was described in the above-noted '759patent with the specific use with a short pulse Q-switched system. Ifdesired this device could also be placed at the output of the fibersystem to shift the output wavelength to a desirable region to improveabsorption contrast, for example. Recognizing the importance of pulsewidth and small spot size requirements for processing, as taught in theabove-noted '759 patent, typical operation of the preferred system formetal link processing will be in the range of about 1.06 μm or beyond,with a 1.08 μm wavelength, for example.

The output of the seed laser is to be amplified for laser materialprocessing. The preferred fiber optic laser amplifier will provide gainof about 30 db. The seed laser output is coupled to the core of thefiber laser either directly or with bulk optics which splits the beamfor fiber delivery. Both techniques are routinely practiced by thoseskilled in the art of ultrafast lasers using chirped pulseamplification, but the system of the preferred embodiment is overallmuch less complex than such ultrafast systems. In the system of thepresent invention, the seed pulse is amplified and no optics for pulsestretching and compression are required. The fiber used in the amplifiersystem is cladding pumped with a diode laser having a substantiallydifferent wavelength than the seed laser, for example 980 nm, whichallows for optical isolation of the seed and pumping beams with adichroic mirror in the bulk optical system arrangement. From thestandpoint of cost, size, and ease of alignment, the preferredarrangement utilizes a coupling arrangement where the seed laser isdirectly coupled to the fiber amplifier. The pump laser injects the highpower diode energy, say at 980 nm wavelength, into the claddingstructure of a rare earth Ytterbium (Yb)-doped fiber using couplingtechniques familiar to those skilled in the art of fiber laser systemdesign.

Low distortion is an important characteristic of the fiber amplifier.Low distortion allows the output pulse shape to substantially match theseed laser pulse shape or possibly further enhance the pulse edges oruniform power shape. The fiber optic gain medium produces the amplifierpulse of FIG. 5 which is delivered to the optical system and focusedonto the object.

Multiple fiber amplifiers can be cascaded for further gain if desired,provided the distortion is low. It could be advantageous to provideactive optical switches or passive optical isolators at the output ofintermediate stages to suppress spontaneous emission. These techniquesare known by those skilled in the art and are disclosed in U.S. Pat. No.5,400,350 and WO 98/92050, for example.

In some cases it may be desirable to further improve the pulse shape byreducing the “tails” with a pulse slicer added to the laser sub-system.This may be in the form of an electro optic device such as a pockelscell or preferably a low delay acousto-optic modulator. This techniquecan suppress energy in the pulse tails to negligible levels whenever therisk of damage occurs at a small multiple of the “pulse duration” of theprocessing pulse. For example, if the energy is reduced by 20 dB (100:1)within 1.5 times the predetermined pulse duration, there will be for allpractical purposes no risk of substrate damage in metal link blowingapplications. To be more specific, if a pulse duration of 8 ns is chosenfor a square pulse shape in a metal link blowing application and theenergy is 20 dB down at 12 ns, the remaining energy is far below thatwhich would cause damage to the Si substrate, this damage beingsubstantial at about 18 ns or more after application of the laser pulse.In a preferred mode of operation, the low delay, high bandwidth pulseslicer will be activated near the end of the amplifier pulse durationand will have a multiplicative effect on the pulse tail, with minimaldistortion of the central lobe. Any effects of the amplifier distortionand the “turn on delay” of the modulator can be compensated to somedegree by changing the shape of the seed diode laser waveform during thepulse duration. The resulting temporal pulse shape in the focused beamis compensated and is of the desired square shape.

Also, presently fiber systems operate optimally at pulse repetitionrates of about 20 KHz which is somewhat faster than the processing rate.An output optical switch, for example a low delay acousto-opticmodulator, with its driver operatively connected to a computer, selectpulses for processing. In this way the reliability of the fiberamplifier and hence the processing system is high. Those skilled in theart will recognize that it would be advantageous from an economicalstandpoint to combine the pulse slicer and output optical switch into asingle module.

Laser System—Alternative

There are numerous advantages cited above of the preferred system of theseed laser and fiber amplifier. Current modulation of the laser diodewith an appropriate driver can directly produce a desired gain-switchedpulse shape which is amplified by the fiber laser amplifier with lowdistortion. The method is contemplated as the best and most efficientapproach to practicing the present invention. However, those skilled inthe art of laser pulse generation and shaping will recognize that otherless efficient approaches can be used. For example, modifications ofQ-switched systems extending beyond the teachings of U.S. Pat. No.4,483,005 are possible to obtain relatively flat pulses by using variouscontrol functions to drive a pockels cell or optical switches providedthat the modulator response time is fast enough. Modern techniques foreffecting pulse width include the use of modified output couplers, forinstance, replacing conventional glass in Nd:YAG Q-switched lasers withGaAs, in either bulk or crystal form. Q-switched pulse of duration fromseveral picoseconds to a few nanoseconds have been reported in passiveQ-switching of an Nd:YAG laser with a GaAs output coupler, OPTICALENGINEERING, 38(11), 1785-88, November 1999.

Laser Processes Steps and Results

The metal link 25 is supported on the silicon substrate 30 by silicondioxide insulator layer 32, which may be, e.g., 0.3-0.5 microns thick.The silicon dioxide extends over the link, and often an additionalinsulating layer of silicon nitride is present over the SiO, layer. Inthe link blowing technique, the laser beam impinges on each link andheats it to the melting point. During the heating, the metal isprevented from vaporizing by the confining effect of the overlyingpassivation layers. During the duration of the short pulse, the laserbeam progressively heats the metal, until the metal so expands that theinsulator material ruptures. At this point, the molten material is undersuch high pressure that it instantly vaporizes and blows cleanly outthrough the rupture hole.

As disclosed in the above-noted '759 patent, with the very small spotsize used with small metal links, the heat may be considered to spreadin essentially an exponential gradient by conduction from the portion ofthe beam striking the target. By employing a peak beam power so highthat sufficient energy for evaporation of the link is delivered in apulse of 8 nanoseconds, and preferably substantially less, theconductive component of heat transfer can be substantially confined to ametal link and the underlying oxide layer, despite its being very thin,such that the temperature rise in the silicon attributable to conductionand the temperature rise attributable to absorption of the beam insilicon, can cumulatively be kept below the temperature threshold atwhich unacceptable silicon damage occurs.

Furthermore, the above-noted '759 patent teaches several importantaspects related to the thermal transfer characteristics of the link andadjacent structures. A thermal model predicts that narrow pulse widths,3-10 ns, for example, which in turn are dependent upon the thickness ofthe target materials, are preferred to avoid heat conduction andsubsequent damage to the Si substrate for representative dimensions.However, it is critically important to realize that other structuresadjoining the link can also affect the quality of laser processingresults, as the following experimental results indicate.

The benefits of the gain-switched, square pulse shape were verified withboth experimental results on and through computer simulation (finiteelement analysis). Specifications for the laser used for link blowingwere:

-   -   Laser wavelength 1.083 microns    -   Maximum Laser energy 10 microjoules    -   Pulse width 7 ns (FWHM, square pulse)    -   Repetition rate 10 KHz (70 KHz laser rate)    -   Spatial profile Gaussian, TEM-00, M²=1.02 (times diffraction        limit)    -   Polarization Unpolarized    -   Pulse Rise Time ˜0.5 ns

The laser of choice was a Ytterbium, cladding pumped fiber laser, in theMOPA configuration using a 980 nm pump diode and a 7 micron diametersingle mode fiber.

Experimental results with the laser specified above on recent generationmemory devices demonstrated superior performance when compared to thestandard Q-switched laser systems. The results led to a conclusion thatthe short, fast rising pulse of the MOPA laser accounted for thesuperior performance. As disclosed earlier, the reasons are threefold:

-   -   1. The 1.083 wavelength is long enough to avoid substrate        damage—about 10 times less absorption occurs at 1.083 μm        compared to the 1.047 μm wavelength.    -   2. The fast rising pulse provides a thermal shock to the        overlying layer of oxide which facilitates link removal.    -   3. The high power density of the fast rising pulse reduces the        link reflectivity which allows for efficient energy coupling.

These characteristics provide a significant departure from theinteraction observed with Q-switched systems. Furthermore, a computerfinite element model was used to simulate the effects of the fast risingpulse for various material thickness and link sizes. The resultsindependently confirmed the improved link blowing results with the useof a sharp rise time pulse with an approximate square distribution. Theresults of the computer model generated by Bernstein, author ofreference number 3, are shown in FIGS. 11 a and 11 b. The followingTables A and B are associated with the graphs of FIGS. 11 a and 11 b,respectively: TABLE A Model 1 @ 0.7 uJ Square Pulse Slow Rise Pulse1^(st) Crack 929K @ 1.88 ns 978K @ 2.40 ns 2^(nd) Crack 1180K @ 2.93 ns1380K @ 3.45 ns 3^(rd) Crack 1400K @ 2.05 ns No 4^(th) Crack 1520K @4.73ns No A1 thickness: 0.8 μm A1 width: 0.8 μm SiO₂: 0.1 μm Si,₃N₄: 0.4 μmLaser energy: 0.7 uJ

TABLE B Model 2 @ 0.7 uJ Square Pulse Slow Rise Pulse 1^(st) Crack 974K@ 2.03 ns 1050K @ 2.55 ns 2^(nd) Crack No No 3^(rd) Crack No No 4^(th)Crack No No A1 thickness: 0.8 μm A1 width: 0.8 μm SiO₂: 0.6 μm Si,₃N₄:0.6 μm Laser energy: 0.7 uJ

The stress and temperature history indicate with certainty theimportance of the fast rising pulse, with sub-nanosecond rise time. Itis also known that if significant pulse energy is present, severalnanoseconds after the ablation is completed, say at 15 ns, the Si can bedamaged. A fast fall time, with a high extinction, is also important.

According to the invention, the silicon substrate is also keptrelatively cool both by appropriate selection of wavelength and bylimiting the pulse duration, with a correspondingly square pulse withfast decay. The laser wavelength in this example is slightly less thanthe room temperature absorption edge of silicon (about 1.1 μm). Althoughthe results reported here did not indicate substrate damage, it shouldbe noted that improved margins are available if desired. For example,the Raman shifter could be utilized to shift the output wavelengthbeyond the absorption edge. Alternatively, another diode laserwavelength could potentially become commercially available for a MOPAconfiguration. Such wavelength selection and shifting techniques mayadvantageously be utilized in other laser processing and micromachiningapplications. In any case, by thus limiting the heating, it is possibleto ensure that the silicon does not shift its absorption edge into theinfrared and enter a thermal runaway condition in which silicon damagecan occur.

The specific embodiment of the MOPA configuration for fast pulsegeneration for cleanly blowing metal links is taken as an example ofpulse shaping and is provided to be illustrative rather thanrestrictive. Through direction modulation of the seed laser, excellentsub-nanosecond control over the pulse shape was maintained, and found tobe advantageous, including the possibility of fast compensation tocorrect the output pulse shape. Other applications in micromachining,marking, scribing, etc. could also benefit from precise, fast pulsecontrol. For example, the seed diode could as easily be modulated with a“sawtooth” waveform or other non Q-switched waveshape for the purpose ofcreating or removing a specific feature on or within a surface.Likewise, because of the fast response of the laser diode, it ispossible to generate a sequence of variable width, short pulses in rapidsuccession. Those skilled in the art of laser processing will recognizethe broad application of the laser system herein. The scope of theinvention is indicated by the following claims and is not to beotherwise restricted.

1. A system for removing target material associated with electricallyconductive links positioned between respective pairs of electricallyconductive contacts in a circuit fabricated on a substrate, said linksdefining a link width, said system comprising: a laser for generating apulse train of laser pulses including at least two closely spaced laseroutput pulses during a first time interval; an optical switch configuredto select from the pulse train of laser pulses said at least two-closelyspaced laser output pulses, said at least two-closely spaced laseroutput pulses being a sequence of processing pulses for focusing onto aspot on the substrate, said at least two-closely spaced laser outputpulses produced in rapid succession at a repetition rate exceeding asystem link processing rate, each of the at least two closelytime-displaced laser output pulses defining a laser spot size largerthan the link width and having energy characteristics which preventundesirable damage to a passivation layer beneath the link; a beampositioning system imparting relative movement of a laser spot positionover the substrate in response to processing control signalsrepresenting one or more locations of electrically conductive links; acontroller for coordinating the optical switch and the beam positioningsystem such that the relative movement is substantially continuous andthe at least two closely-spaced laser output pulses impinge a firstelectrically conductive link at a first location between first contactsand encompass the link width, the at least two closely spaced laseroutput pulses removing target material without causing undesirabledamage to the substrate or a layer of insulating material below thelink.
 2. The system of claim 1, wherein each of the laser output pulseshaving a pulse duration in the range of several picoseconds to about tennanoseconds.
 3. The system of claim 1, wherein said laser comprises apumped, mode locked, pulsed laser.
 4. The system of claim 1, whereinsaid laser comprises a seed laser diode and a fiber amplifier.
 5. Thesystem of claim 2, wherein said laser comprises a seed laser diode and afiber amplifier.
 6. A laser system for employing laser output to removetarget material from locations of selected link structures electricallyconductive links positioned between respective pairs of electricallyconductive contacts in a circuit fabricated on a substrate, said linksdefining a link width, said system comprising: a pumped, mode lockedlaser for emitting a pulse train of laser output pulses; an opticalgating device to select a sequence of pulses from said laser outputpulses comprising at least two closely time-displaced laser outputpulses for focusing onto a spot on the substrate, said at leasttwo-closely time-displaced laser output pulses produced in rapidsuccession at a repetition rate exceeding a system link processing rate,each of the at least two closely time-displaced laser output pulsesdefining a laser spot size larger than the link width and having energycharacteristics which prevent undesirable damage to the substrate and toany passivation layer beneath the link; a beam positioning systemimparting relative movement of a laser spot position to the substrate inresponse to processing control signals representing one or morelocations of electrically conductive links; a laser system controllerfor coordinating operation of the optical gating device and the beampositioning system such that the relative movement is substantiallycontinuous and the at least two closely time-displaced laser outputpulses impinge a first electrically conductive link at a first locationbetween first contacts and encompass the link width, the at least twoclosely time-displaced laser output pulses removing target material atthe location of the selected link structure without causing damage tothe substrate or layer of insulating material below the link.
 7. Thesystem of claim 6, wherein said laser comprises a Q-switched, solidstate laser and the gating device comprises an optical switchpositioned, external to the laser.
 8. The system of claim 6, furthercomprising an amplifier for amplifying the laser output pulses.
 9. Thesystem of claim 6, wherein the pulses sever the link.
 10. The system ofclaim 6, wherein the link is covered with an overlying passivationlayer, and the pulses remove the layer.
 11. The system of claim 6,wherein the link is covered with an overlying passivation layer, and thepulses remove the layer and sever the link.
 12. The system of claim 6,wherein said link comprises metal, polysilicide, or polysilicon.
 13. Thesystem of claim 6, wherein the underlying layer comprises an underlyinglayer of SiO₂.
 14. The system of claim 6, wherein links are processed ata rate of at least 1 kHz to somewhat less than about 20 kHz.
 15. Thesystem of claim 6, wherein each of the laser pulses has a pulse width inthe range of several picoseconds to about ten nanoseconds.
 16. Thesystem of claim 6, wherein each of the laser pulses has a pulse width ofless than 5 nanoseconds.
 17. The system of claim 6, wherein each of thepulses produces a focused laser spot having a power density of at least10⁹ W/cm2, a diameter of about 1 to 4 micrometers, and duration ofseveral picoseconds to about 10 nanoseconds, whereby said pulses have anenergy ranging from about 0.1 nanojoules to about 5 microjoules.
 18. Thesystem of claim 1, wherein the laser outputs pulses having energy of 0.1microjoules to 3 microjoules, wherein said optical switch attenuates thelaser output pulses from the laser by up to 90% such that the energy ofthe pulses output from the optical switch and impinging the targetmaterial ranges from 10 nanojoules to 3 microjoules.
 19. The system ofclaim 6, wherein each of the pulses has an energy of 0.1 microjoules upto 3 microjoules of energy.
 20. The system of claim 17, wherein each ofthe pulses has an energy of 0.1 microjoules up to 3 microjoules ofenergy.
 21. The system of claim 6, wherein each of the output pulseshave approximately the same energy.
 22. The system of claim 6, whereinat least two of said output pulses have different pulse widths.
 23. Thesystem of claim 22, wherein at least two of the output pulses havedifferent energies.
 24. The system of claim 6, wherein the pulses aregenerated at a wavelength of less than about 2000 nm.
 25. The system ofclaim 6, wherein the beam positioning system causes relative movement atsubstantially constant speed during processing.
 26. The system of claim1, wherein said laser comprises a Q-switched, solid state laser and theoptical switch is positioned external to the laser.
 27. The system ofclaim 1, further comprising an amplifier for amplifying the laser outputpulses.
 28. The system of claim 1, wherein the pulses sever the link.29. The system of claim 1, wherein the link is covered with an overlyingpassivation layer, and the pulses remove the layer and sever the link.30. The system of claim 1, wherein said link comprises metal,polysilicide, or polysilicon.
 31. The system of claim 1, wherein theunderlying layer comprises an underlying layer of SiO₂.
 32. The systemof claim 1, wherein links are processed at a rate of at least 1 kHz tosomewhat less than about 20 kHz.
 33. The system of claim 1, wherein eachof the laser pulses has a pulse width in the range of severalpicoseconds to about ten nanoseconds.
 34. The system of claim 1, whereineach of the laser pulses has a pulse width of less than 5 nanoseconds.35. The system of claim 1, wherein each of the amplified pulses has anenergy of 0.1 microjoules up to 3 microjoules of energy.
 36. The systemof claim 2, wherein each of the pulses produces a focused laser spothaving a power density of at least 10⁹ W/cm², a diameter of about 1 to 4micrometers, and duration of several picoseconds to about 10nanoseconds.
 37. The system of claim 36, wherein said pulses have anenergy ranging from about 0.1 nanojoules to about 5 microjoules.
 38. Thesystem of claim 1, wherein each of the output pulses have approximatelythe same energy.
 39. The system of claim 1, wherein at least two of theoutput pulses have different energies.
 40. The system of claim 1,wherein the pulses are generated at a wavelength of less than about 2000nm.
 41. The system of claim 1, wherein the beam positioning systemcauses relative movement at substantially constant speed duringprocessing.
 42. The system of claim 1, wherein said laser generates asequence of short pulses in rapid succession.
 43. The system of claim 1,wherein said short pulses have variable width.